Ch. 15 & 16: Forces, Motion and Machines

Ch. 15 & 16 Vocab Force: a push or pull Friction: the force that resists the movement of one surface past another Gravitational Force: the force of attraction between the objects in the universe Speed: a measure of how fast an object is moving Velocity: the speed of an object in a particular direction

Vocab Acceleration: the rate at which velocity changes Inertia: the tendency of an object to remain at rest or in constant motion unless a force acts on it Momentum: a measure of the force needed to stop a moving object Work: to use force in order to move an object a certain distance Machine: any device that helps people do work

Vocab Simple Machine: a tool made up of one or two parts Compound Machine: a machine made up of one or more simple machines Fulcrum: a support on which a lever rests while moving of lifting an object Load: force of an object on a lever Effort Force: a force applied to the end of a lever to lift a load

Ch What happens when forces act on objects?

Forces A force has both size and direction Some forces act only if objects touch Hands pushing a box Other forces act even if objects don’t touch Earth’s gravity pulls you when you jump Magnets push or pull each other

Measuring Forces Forces are measured in units called newtons (N) One newton is the force needed to change the speed of a one-kilogram object by one meter per second each second It takes about one newton of force to lift a small apple Force can be measured using a spring scale Attach an object to one end of the scale and hold it up The spring inside stretches to show the force needed to support the object

Forces on Objects There are more than one force acting on an object at a time Flying a kite Weight of kite pulls it down Force of wind pushes it up You guide the kite by applying force on the string The combination of all the forces acting on an object determines the effect of the forces Balanced forces cause no change in motion, even if the object is moving Unbalanced forces can cause an object at rest to move Can also change the speed or direction of a moving object

Forces on Objects To find the overall effects of forces acting on an object, you add the forces together The resulting force is called the net force Net force doesn’t always determine the direction that an object moves It determines the change in an object’s motion EX: there is a 5N force pulling an object to the right while a 3N force pulls to the left Net force is 2N to the right

Friction There are 3 types of friction Rolling: resists the motion of a rolling object EX: resists the motion of a skateboards wheels Sliding: resists the motion of a sliding object EX: pushing a box across the floor Static: resists the motion of an object just as it begins to move EX: first trying to push the box

Friction Friction depends on the type of surfaces rubbing against each other and how strongly they are pushed together Friction is mainly caused by rough surfaces Even surfaces that appear smooth have tiny bumps and holes Friction is usually greater for rough surfaces If both surfaces are very smooth and flat, the attraction of their particles increases their friction A soft or rubbery surface also has more friction because it easily bends

Helpful and Harmful Friction Friction is useful for many things Walking across a room would be like walking on ice without friction Friction of the brake pads on the brake drum helps slow down a car Friction can also be harmful Heat produced by friction makes engines from less efficiently Friction between wind and soil can cause erosion Also wears away the rubber on tires and form holes in the soles of shoes

Ch How does gravity affect objects?

Gravitational Force Earth’s gravity pulls all objects on Earth toward Earth’s center Gravitational force holds water in the ocean and holds the air near the Earth It also affects the way plants grow and the way your bones develop Isaac Newton was the first to explain the many details of gravity Realized that gravity depends on the masses of the two objects that exert gravitational forces on each other

Gravitational Force The greater the mass of an object the stronger its gravitational force Astronauts on the Moon experience a lower gravitational force because the Moon has less mass than Earth Gravitational force depends on the distance between two objects As distance increases, gravitational pull decreases

Gravity and the Universe The gravitational forces that impact us on Earth, also occur in the solar system We don’t feel the Sun’s gravitational pull because we are farther away from its center The closer to the center of an object you are, the stronger the gravity

Tides A tide is a rise or fall in water level near the ocean’s shore Most coastal areas around the world have two high tides and two low tides a day The Moon’s gravity pulls on the ocean The part of the Earth facing the Moon has a high tide so does the opposite side of the Earth The other parts of Earth experience a low tide

Ch How can you describe motion?

Observing Motion In order to determine if another object is moving, you must view it in relation to another object that appears to stay in place This is called your frame of reference How would you describe your motion right now?

Kinds of Motion Circular Motion: takes place around a central point Straight Line Motion: motion that takes place in a straight line Vibrational Motion: a rapid back and forth movement

Calculating Speed You can find speed by dividing the distance traveled by the time needed to move that distance Often measured in kilometers per hour Average speed= distance/time An object’s speed at any time is its instantaneous speed

Velocity When an object changes direction its velocity changes Even if the speed stays the same, the velocity is different Even going around a curved road at the same speed changes the velocity

Acceleration The motion of moving objects usually isn’t constant The instantaneous speed and direction of motion may change Acceleration isn’t just an object going faster It can also happen when an object slows down or changes direction Velocity can only change if a force acts on the object Acceleration is therefore the result of unbalanced forces acting on the object

Ch What are the laws of motion?

First Law of Motion Isaac Newton came up with 3 laws of motion An object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line, unless acted on by an unbalanced force If you put a book on a table it wont move until you move it Throw a baseball and it will keep moving This first law is sometimes called the law of inertia The amount of inertia an object has depends on its mass The greater the mass the greater its inertia

Second Law of Motion The acceleration of an object depends on the mass of the object and the size of the net force applied

Using an Equation The second law of motion can be written as an equation Acceleration= force/mass Force: net force applied to the object Mass: the object’s mass Acceleration: object’s acceleration Not all forces cause a change in motion Only unbalanced forces cause an object to accelerate

Third Law of Motion When a force is applied to an object, the object exerts an equal force in the opposite direction

Momentum

Ch How do machines help people work?

Measuring Work Work in science has a different meaning than what is used in everyday life Work means using force, pushing or pulling, in order to move an object a certain distance No matter how much force you use, if the object doesn’t move you haven’t done work To find out how much work is done, use this formula Work = force X distance Work measured in a unit called the joule(j) 1 joule (j) = 1 newton (N) X 1 meter (m)

Work and Machines There are many different types of machines Some can be as simple as chop sticks Others are complicated and can have thousands of parts

How Machines Help Every machine makes some kind of tradeoff that helps in some way Work is equal to force times distance EX: wheelbarrow moving dirt Car jack lifting a car

Machines and Friction In order for a machine to do its job, work must go into it A person using the machine or a battery is providing the work The amount of work done by a machine is less than the amount put into it This is because of friction Heat is often a result of friction Ways to reduce friction include using lubricants, wheels, or rollers

Ch What are types of simple machines?

Lever

Inclined Plane An incline plane is a slanted surface An inclined plane reduces the force needed to move the load, but the load travels farther up the ramp You are doing the same amount of work either way

The Wedge

The Screw A screw is a type of inclined plane wrapped in a spiral

Wheel and Axle

Pulley

Compound Machines